The present invention relates to a pattern configuration to be employed to evaluate the aberration characteristics of a reducing projection lens of a stepper which is utilized to form a pattern on a semiconductor substrate during a photolithography step implemented in a semiconductor device manufacturing process.
During the photolithography step implemented in a semiconductor device manufacturing process, a reducing projection exposure apparatus called a stepper is employed to repeat a step for transferring a circuit pattern formed on a reticule mask onto a semiconductor substrate (hereafter referred to as a wafer) to manufacture a semiconductor device. In the prior art, the aberration characteristics components of the reducing projection lens of the stepper are evaluated in order to ensure that the shape of the circuit pattern is accurately transferred during the photolithography step. Various types of testing and measurement are performed in correspondence to the specific aberration components of the lens during the stepper lens aberration characteristics evaluation.
As an example, coma aberration measurement is explained. If a coma aberration is present, convergence spots become asymmetrical, which causes asymmetry in the transferred pattern. FIG. 20 illustrates a pattern configuration employed in an evaluation in the prior art. It is constituted of a line-and-space pattern achieved by a repeating arrangement of several lines and spaces having a width corresponding to the resolving power of the stepper lens. The width in this context refers to the length of the shorter side of the lines and spaces in the figure. In FIG. 20(a), the widthwise direction extends along the horizontal direction. In FIGS. 20(b), (c) and (d) respectively, the line-and-space pattern is rotated by 90 degrees, 45 degrees in the clockwise direction and 45 degrees in the counterclockwise direction relative to the state illustrated in FIG. 20(a).
In the prior art, this pattern configuration is set at various exposure positions of the lens, as illustrated in FIG. 21, the widths of the outermost patterns in each pattern configuration are measured and the difference in the measured widths is evaluated as the coma aberration at the corresponding exposure position. An outermost pattern refers to a pattern that does not have another pattern lying adjacent to it on its outside within the same pattern configuration. This measurement method is based upon the principle that in line-and-space patterns, the shapes of the edges of patterns having other patterns lying adjacent to them are less likely to be affected by aberrations compared to outermost patterns.
For instance, the pattern configuration shown in FIG. 20(a) is used to ascertain the coma aberration component along the horizontal direction. The dimensions of XL and XR which are the outermost pattern widths among the plurality of lines and spaces transferred and formed on the wafer are measured by using an SEM length measuring machine and then the coma aberration is calculated through:
coma aberration=(XL)xe2x88x92(XR) or
coma aberration=((XL)xe2x88x92(XR))/((XL)+(XR)).
This measurement and calculation process is implemented at each exposure position of the lens shown in FIG. 21 to ascertain the coma aberration at each position.
The upper outermost pattern width YU and the lower outermost pattern width YL are measured and their difference is calculated to ascertain the coma aberration component along the vertical direction by using the pattern configuration in FIG. 20(b). Likewise, the pattern widths +45L and +45R are measured and their difference is calculated as the coma aberration component along the diagonal direction extending from the upper left to the lower right by using the pattern configuration in FIG. 20(c). The pattern widths xe2x88x9245L and xe2x88x9245R are measured and their difference is calculated as the coma aberration component along the diagonal direction extending from the upper right to the lower left by using the pattern configuration in FIG. 20(d). Thus, by using the four types of pattern configurations shown in FIG. 20 set at the individual exposure positions and measuring the transferred patterns formed by them, the coma aberrations manifested in the various directions at each exposure position can be evaluated.
Next, as another example of aberration measurement, measurement of astigmatism is explained. If an astigmatism is present, the correct focus position changes in correspondence to the direction in which a pattern is formed. For instance, if the exposure surface is set at the correct focus position for a given transferred pattern, another transferred pattern extending along a direction perpendicular to the first transferred pattern becomes defocused. Thus, the widths of the pre-transfer patterns that are equal to each other become different when they are transferred along two different directions at the exposure surface. Based upon this concept, the dimensional difference between transferred patterns along different directions at a given exposure surface is measured and calculated and then evaluated as the astigmatism.
In more specific terms, the pattern width 0C at the center of the pattern structure in FIG. 20(a) and the pattern width 90C at the center on the pattern structure in FIG. 20(b) are measured and the 0xc2x0 directionxe2x80x9490xc2x0 direction astigmatism is calculated and evaluated as:
0xc2x0-90xc2x0 direction astigmatism=(0C)-(90C).
Likewise, 45C in FIG. 20(c) and 135C in FIG. 20(d) are measured and the 45xc2x0 directionxe2x80x94135xc2x0 direction astigmatism is calculated and evaluated as:
45 degrees 135 degrees direction astigmatism=(45C)-(135C). As an overall astigmatism quantity, the dimensional difference is calculated through:
astigmatism=MAX ((0C), (90C), (45C)), (135C))xe2x88x92MIN ((0C), (90C), (45C)), (135C))
for each pattern configuration on the same exposure surface by comparing the pattern widths along the 0 degree direction, the 90 degrees direction, the 45 degrees direction and the 135 degrees direction, and the dimensional difference thus ascertained is evaluated as the astigmatism. As in the coma aberration measurement explained earlier, this process of dimensional measurement, comparison and calculation is performed for the pattern configuration at each of the exposure positions set within the lens exposure range, so that the astigmatism can be evaluated in correspondence to each exposure position. The central width in each pattern structure is used in the astigmatism measurement since the edge shapes of patterns adjacent to other patterns are assumed to be affected by coma aberration to a lesser degree compared to outermost patterns.
In the measuring method using the patterns described above, a coma aberration is calculated by measuring the dimensional difference between the outermost patterns positioned symmetrical to each other. However, even when a pattern is affected by a coma aberration, a dimensional difference does not always manifest itself between the widths of the outermost patterns. FIG. 22(a) is a sectional view of the pattern formed by using the pattern configuration shown in FIG. 20(a). Even when the pattern is affected by an aberration, as illustrated in FIG. 22(a), LE and RI may be very close to each other (LExc2x7RI) and, as a result, no significant dimensional difference may be manifested. In other words, the measuring method in the prior art has a problem in that an aberration component may not be measured accurately.
FIG. 22(b) is a cross section of one of the line patterns that have been formed. The coma aberration manifests itself as a difference between the left and right edge shapes within a given pattern as shown in FIG. 22(b) and should, therefore, be evaluated as the dimensional difference ERxe2x88x92EL. However, since the dimensional difference between the left and right edges of the same pattern is not ascertained in the measuring method in the prior art, there is a problem in that the evaluation is not performed accurately.
In addition, since the evaluation is performed with the pattern size set close to the limit of the resolving power of the stepper in the measurement method employed to measure coma aberration and astigmatism in the prior art, an SEM length measuring machine is employed to measure pattern dimensions. Since an SEM length measuring machine performs measurement at a high magnification factor, it is difficult to measure a plurality of patterns within a given field of view. Thus, it is necessary to repeat the measuring process by moving the measuring position to measure each new pattern. As a result, the aberration evaluation process described earlier, which necessitates numerous patterns to be measured, becomes complicated and time consuming.
An object of the present invention, which has been completed by addressing the problems of the prior art discussed above, is to provide a stepper lens aberration measurement pattern and a stepper lens aberration characteristics evaluating method that make it possible to perform a stepper lens aberration characteristics evaluation with a high degree of sensitivity without requiring a great length of time by employing an optical length measuring machine.
In order to achieve the object described above, in a first aspect of the present invention, a pattern to be used for stepper lens aberration characteristics evaluation includes line-and-space type first patterns, each first pattern constituted of a plurality of line patterns each having a width along the direction of its shorter side set at a dimension that does not allow separation/resolution in the field of view of an optical length measuring machine, roughly rectangular second patterns having external dimensions that allow separation/resolution in the field of view of the optical length measuring machine arranged to provide at least two joint patterns each formed by joining a first pattern and a second pattern so as to roughly resemble a comb, in which the joint patterns are placed over a distance from each other that is set to allow separation/resolution in the field of view of an optical length measuring machine while achieving a positional relationship whereby line portions of their first patterns extending outward are symmetrical to each other, is provided. When this pattern is transferred and formed on a wafer by employing a stepper, the dimensions of the symmetrical portions of the pattern can be measured with the optical length measuring machine so that the coma aberration component of the stepper lens can be evaluated with a high degree of sensitivity through comparison/calculation.
It is to be noted that by using the joint patterns described above, a joint pattern set, which is achieved by combining a pair of joint patterns over a distance from each other that is set to allow separation/resolution in the field of view of an optical length measuring machine with the line portions of their first patterns extending outward achieving a symmetrical relationship with each other with at least one identical joint pattern pair, may be prepared. By transferring and forming this joint pattern set onto a wafer, the coma aberration component of the stepper lens can be evaluated with a high degree of sensitivity through measurement of distances between the front ends along the lengthwise direction in the first patterns at the two sides of each joint pattern pair.
In a second aspect of the present invention, a pattern for stepper lens aberration characteristics evaluation, comprising a pattern set achieved by placing a third pattern having external dimensions that allow separation/resolution in the field of view of an optical length measuring machine and having a side parallel to the inner side of each second pattern in the joint pattern set described above in an inner space enclosed by the second patterns in the joint pattern set over distances from the second patterns that allow separation/resolution by the optical length measuring machine with sides of the third pattern extending parallel to the inner sides of the individual second patterns, is provided. By transferring and forming this pattern set onto a wafer, the degree to which the edges of the rectangular patterns undergoing measurement are affected by an aberration can be reduced. The shape of the third pattern may be rectangular, hexagonal or octagonal in correspondence to the number of joint pattern sets provided around it.
In a third aspect of the present invention, a pattern to be used for stepper lens aberration characteristics evaluation achieved by rotating the joint pattern set and the pattern set described above by the same degree is provided. For instance, by setting them rotated by 45 degrees, it becomes possible to evaluate the coma aberration component along the 0 degree direction, the +45 degrees direction, the xe2x88x9245 degrees direction and the 90 degrees direction. In addition, by setting the joint patterns in four directions, measurement and evaluation of the coma aberration component in two directions, i.e., the 0 degree direction and the 90 degrees direction, can be achieved with a single pattern. Furthermore, by positioning the joint patterns in eight directions, measurement and evaluation of the coma aberration components in four directions, i.e., the 0 degree direction, the +45 degrees direction, the xe2x88x9245 degrees direction and the 90 degrees direction can be achieved with a single pattern.
In a fourth aspect of the present invention, a pattern to be used for stepper lens aberration characteristics evaluation includes line-and-space type first patterns each constituted of a plurality of line patterns that each have a width along the direction of its shorter side set at a dimension that does not allow separation/resolution in the field of view of an optical length measuring machine, a second pattern having external dimensions that allow separation/resolution in the field of view of an optical length measuring machine and sides facing opposite each other extending parallel to each other, a roughly rectangular fourth pattern having external dimensions that allow separation/resolution in the field of view of an optical length measuring machine. A pattern set is achieved by joining the first patterns to at least two sides of the second pattern facing opposite each other and placing the fourth pattern over a distance that allows separation/resolution in the field of view of an optical length measuring machine from the front end of each first pattern along the direction of the length of the lines. By transferring and forming this pattern onto a wafer, the pattern dimensions can be measured by the optical length measuring machine and the astigmatism component of the stepper lens can be evaluation with a high degree of sensitivity through comparison/calculation.
In a fifth aspect of the present invention, a pattern to be used for stepper lens aberration characteristics evaluation comprising a pattern set achieved by joining line-and-space type fifth patterns each constituted of a plurality of line patterns each having a width along the direction of its shorter side set at a dimension that does not allow separation/resolution in the field of view of the optical length measuring machine with the outer sides of the fourth patterns is provided. By using this pattern set, various aberration components can be measured with a single pattern set.
In a sixth aspect of present invention, a pattern to be used for stepper lens aberration characteristics evaluation achieved by rotating the pattern set described above by the same degree is provided. By setting them rotated by 45 degrees each, the astigmatism components along the 0 degree xe2x88x9290 degrees direction and the 45 degrees direction can be evaluated.
It is to be noted that the line patterns described in reference to the first through sixth aspects may be formed in a rough wedge shape with their width at the bottom sides along the direction of their shorter sides set at a dimension that does not allow separation/resolution in the field of view of the optical length measuring machine. In addition, the width of the line patterns in the direction of their shorter sides mentioned in reference to the first through sixth aspects may be set equal to or smaller than the stepper resolution limit. Furthermore, the outermost dimensions of the joint pattern sets and the pattern sets described in reference to the first through sixth aspects may be set at dimensions that can be contained within the measurement field of view of the optical length measuring machine.
In a seventh aspect of the present invention, a stepper lens aberration characteristics evaluating method comprises a step in which an aberration characteristics evaluation pattern constituted of a pattern set achieved by setting at least two joint patterns, each formed by joining a line-and-space type first pattern constituted of a plurality of line patterns each having a width along the direction of its shorter side set at a dimension that does not allow separation/resolution in the field of view of an optical length measuring machine with a roughly rectangular second pattern having external dimensions that do not allow separation/resolution in the field of view of an optical length measuring machine in a rough comb shape over a distance from each other set at a dimension that allows separation/resolution in the field of view of an optical length measuring machine while achieving a positional relationship whereby line portions of the first patterns extending outward are symmetrical to each other is transferred onto an evaluation substrate by using a stepper. This method also includes a step in which an optical length measuring machine is utilized to measure the distance between the front end portion of the first pattern in the lengthwise direction and the edge of the second pattern on the opposite side from the first pattern in one of the joint patterns transferred onto the evaluation substrate. In another step, the distance between the front end portion of the first pattern along its lengthwise direction and the edge of the second pattern on the opposite side from the first pattern in another joint pattern positioned symmetrical to the one joint pattern is measured. Then a step is performed in which the dimensional values obtained through the measurement are compared to obtain their difference through calculation.
It is to be noted that the joint pattern set may include a third pattern having external dimensions that allow separation/resolution in the field of view of an optical length measuring machine and having a side extending parallel to the inner side of each of the second patterns, placed in an inner space enclosed by the second patterns in the joint pattern set over a distance that allows separation/resolution in the field of view of an optical length measuring machine from the second patterns.
In an eighth aspect of the present invention, a stepper lens aberration characteristics evaluating method comprises a step in which an aberration characteristic evaluation pattern constituted of a pattern set is achieved by joining line-and-space type first patterns each constituted of a plurality of line patterns each having a width along the direction of its shorter side set at a dimension that does not allow separation/resolution in the field of view of an optical length measuring machine with at least two sides facing opposite each other of a second pattern having external dimensions that allow separation/resolution in the field of view of the optical length measuring machine and sides facing opposite each other extending parallel to each other, and by placing fourth patterns each having external dimensions that allow separation/resolution in the field of view of the optical length measuring machine and formed in a roughly rectangular shape over a distance that allows separation/resolution in the field of view of the optical length measuring machine from the front ends of first patterns along the direction of the length of the line patterns is transferred onto an evaluation substrate by utilizing a stepper. In a subsequent step, an optical length measuring machine is utilized to measure the distance between the front ends of the first patterns along the lengthwise direction set on the two sides of the pattern set transferred onto the evaluation substrate.
It is to be noted that in the method described above, line-and-space type fifth patterns each constituted of a plurality of line patterns each having a width along the direction of its shorter side set at a dimension that does not allow separation/resolution in the field of view of an optical length measuring machine may be joined at the outer sides of the fourth patterns in the pattern set. Then, a step in which the distances between the front ends of the fifth patterns in the lengthwise direction and the edges of the fourth patterns on the opposite sides from the fifth patterns are measured and evaluated may be performed.